tools: add dev_t_major/minor

tools: add dev_t_major/minor

Extract major/minor from `dev_t` values as found in stat()
calls, with support for the full 32 bit values.

The device value returned by stat() is 32 bits long and encoded as
high 12 bit of the minor value as in the 12 MSBs, 12 bit major value,
then the low 8 bit of the minor value in the low byte.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>